Recently, from the viewpoint of effective utilization of energy aimed at global environmental conservation and effective utilization of resources, a nighttime power storage system, a home-use distributed electrical storage system based on photovoltaic power generation technology, and an electrical storage system for an electric vehicle, have attracted attention.
In these electrical storage systems, the first requirement is that energy density of the electrical storage elements is high. As one of the more popular electrical storage elements having a high degree of energy density and capable of meeting other storage requirements, lithium ion batteries have been developed and utilized on a large scale.
The second requirement is the capacity for high output. For example, in a combination of a highly efficient engine and an electrical storage system (for example, a hybrid electric vehicle), or in a combination of a fuel cell and an electrical storage system (for example, a fuel-cell electric vehicle), high output discharge characteristics are required from the electrical storage system, in order to achieve sufficient acceleration.
At the present time, as one type of a high output electrical storage element, electric double layer capacitors using activated carbon as an electrode, have been developed, and exhibit both high durability (in particular, cycle characteristics and high temperature storage characteristics), and output characteristics of about 0.5 to 1 KW/L. These electric double layer capacitors are considered to be the optimum electrical storage element in fields where the above-described high output is required; however, the energy density thereof is only about 1 to 5 Wh/L, and output duration time limit their use in practical applications.
On the other hand, a nickel-hydrogen battery that is currently adopted for use in hybrid electric vehicles has the same high output as that of the electric double layer capacitors, and has an energy density of about 160 Wh/L. However, research is being actively carried out to increase energy density and output thereof, as well as to further improve stability at high temperatures and increase durability.
In addition, as with lithium ion batteries, research continues toward realizing higher output.
For example, a lithium ion battery has been developed that is capable of providing a high output of over 3 kW/L, at a depth of discharge (i.e., a value indicating a state of charge in terms of percentage) of 50%; however, a lithium ion battery has been actually designed to suppress energy density equal to or less than 100 Wh/L, even though a lithium ion battery is identically characterized by the highest energy density (higher than 100 Wh/L).
In addition, durability thereof (in particular, cycle characteristics and high temperature storage characteristics) are inferior to that of the electric double layer capacitors. Therefore, in order to have practical durability, the lithium ion battery is usable only in a depth of discharge that is a narrower range than between 0 to 100%. Therefore, usable capacity in practice is reduced, and further research is being carried out to enhance durability.
As other examples, there have been proposed a microporous membrane made of polyolefin (for example polyethylene), having membrane resistance equal to or less than that of a conventional microporous membrane made of polyolefin, together with high porosity (also referred to as “high void content”); and a non-aqueous electrolytic solution-type secondary battery provided with a microporous membrane made of polyolefin (refer to PATENT LITERATURE 1).
Although practical application of the electrical storage element having high output density, high energy density and durability, as described above, has been required, the above-described existing electrical storage element has advantage and disadvantage. Accordingly, a new electrical storage element satisfying these technological requirements has been required, and as a strong candidate thereof, development of the electrical storage element which is referred to as a lithium ion capacitor has been active in recent years.
The lithium ion capacitor is one type of an electrical storage element (i.e., a non-aqueous lithium-type electrical storage element) using a non-aqueous electrolytic solution including a lithium ion-containing electrolyte, and is the electrical storage element carrying out charge-discharge by a non-faradaic reaction based on adsorption/desorption of a negative ion similar to that in the electric double layer capacitor, in a positive electrode, and by a faradaic reaction based on intercalation/deintercalation of a lithium ion similar to that in the lithium ion battery, in a negative electrode.
As described above, in the electric double layer capacitors carrying out charge-discharge by the non-faradaic reaction in both of the positive electrode/the negative electrode, output characteristics is superior but energy density is low. On the other hand, in the lithium ion battery, which is a secondary battery carrying out charge-discharge by the faradaic reaction in both of the positive electrode/the negative electrode, energy density is superior but output characteristics is inferior. A lithium ion capacitor is a novel electrical storage element which can achieve both superior output characteristics and high energy density, by carrying out charge-discharge based on the non-faradaic reaction in the positive electrode and based on the faradaic reaction in the negative electrode.
Applications using the lithium ion capacitor include electricity storage for railways or construction machines, and automobiles. Since operating environments are severe in these applications, it is necessary to have superior temperature characteristics. Specifically, it is necessary to have high input/output characteristics at low temperature, or high cycle life characteristics at high temperature. As such a lithium ion capacitor, for example, there has been proposed a lithium ion capacitor consisting of a positive electrode having an positive electrode active material layer and a positive electrode current collector, a negative electrode having a negative electrode active material layer and a negative electrode current collector, a separator intervening between the positive electrode and the negative electrode, a non-aqueous electrolytic solution, and an outer casing, characterized in that the positive electrode active material layer contains an activated carbon, the negative electrode active material layer contains a carbon material capable of intercalation of a lithium ion, membrane thickness of the separator is 15 μm to 50 μm, and fluid resistance of the separator is 2.5 Qcm2 or less (refer to PATENT LITERATURE 2)